The introduction of mobile devices, such as cellular phones, PDAs, electronic books, electronic newspapers, portable TV, etc., the use of thin film devices on plastic has dramatically increased. Devices fabricated on plastic are light weight, thinner than glass, and generally unbreakable. The most noticeable progress is the development of organic light emitting diode (OLED) display on plastic substrate was reported by Yoshida et al., 3-inch Full-color OLED Display using a Plastic Substrate, SID Digest, p. 856 (2003), however, the quality and resolution of the disclosed OLED display still requires further improvement to compete with silicon-based thin-film transistor (TFT) technology.
Another approach is to fabricate TFTs directly on plastic substrate, as described by Okada et al., A 4-in Reflective Color TFT-LCD Using a Plastic Substrate, SID Digest, p. 1204 (2002), however, this process requires a fabrication temperature which must be lowered to 220° C. to avoid thermal damage to the plastic. TFT were fabricated using a-Si:H deposited at 220° C., however, the mobility and image quality are much less than a polysilicon TFT.
Polycrystalline-silicon TFTs have been fabricated on a plastic substrate using a film transfer process, as reported by Asano et al., Low-Temperature Polycrystalline-Silicon TFT Color LCD Panel Made of Plastic Substrates, SID Digest, p. 1196 (2002). The reported procedure includes: (1) fabricating the bottom gate TFT device on a glass substrate, with a gate insulating layer and an amorphous silicon (a-Si) layer, (2) the a-Si is crystallized in an excimer laser annealing system, (3) completing the TFT process, (4) gluing the completed TFT to a second, temporary substrate, (5) etching to remove the glass by hydrofluoric acid, (6) affixing a plastic substrate to the rear surface of the TFT with a permanent adhesive, and (7) detaching the temporary substrate.
Wang et al. reported the direct transfer of poly-Si TFT from silicon to a glass or to a plastic substrate, Wang et al., Device Transfer Technology by Backside Etching for Poly-Si Thin-Film Transistors on Glass/Plastic Substrate, Jpn. J. Appl. Phys., 42, L 1044 (2003). The steps in this technique include (1) fabricating a poly-Si TFT on a silicon substrate, (2) gluing the front side to a glass/plastic substrate, and (3) removing the silicon wafer by CMP and wet etching.